In an image-forming process by means of an electrophotographic system, an electrostatic latent image is formed on an inorganic photoconductive material such as selenium, a selenium alloy, cadmium sulfide or amorphous silicon, or on an organic photoconductive material employing a charge-generating material and a charge-transporting material, and the latent image is developed by a toner, then transferred and fixed on a paper sheet or a plastic film to obtain a visible image.
The photoconductive material may be positively electrifiable or negatively electrifiable depending upon its construction. When a printed portion is remained as an electrostatic latent image by exposure, development is conducted by means of an oppositely electrifiable toner. On the other hand, when a printed portion is destaticized for reversal development, development is conducted by means of an equally electrifiable toner. A toner is composed of a binder resin, a coloring agent and other additives. However, in order to impart desired tribocharge properties (such as desired charge up speed, tribocharge level and tribocharge level stability), stability with time and environmental stability, it is common to use a charge-control agent. The properties of the toner will be substantially affected by this charge-control agent.
In a case where a negatively electrifiable photoconductive material is used for development with an oppositely electrifiable toner, or a positively electrifiable photoconductive material is used for reverse development, a positively electrifiable toner is used. In such a case, a positively electrifiable charge-control agent is used.
Further, in a case of a color toner, it is necessary to use a colorless charge-control agent or a charge-control agent with a pale color which does not affect the color of the toner. Such pale-colored or colorless charge-control agents may, for example, be quaternary ammonium chloride compounds disclosed in e.g. Japanese Unexamined Patent Publications No. 119364/1982, No. 9154/1983 and No. 98742/1983.
However, these charge-control agents have various drawbacks such that a charged amount is unstable depending on storing conditions and particularly that a charged amount in a toner which is high at the initial stage of preparation of a developer, is remarkably lowered specially when a temperature and a moisture are high. Further, parahalophenyl carboxylic acid disclosed in Japanese Unexamined Patent Publication No. 186752/1983 has such a drawback that thermostability is poor. Also, the above-mentioned charge-control agents have low electrifying effects or provide oppositely electrifiable toners, or some of them are poor in dispersibility or chemical stability. Thus, none of them has fully satisfactory properties as a charge-control agent.